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How is fuel vapor dealt with at large airports?
When I fill my tank at a gas station, I can see fuel vapor coming out of the tank.
I imagine the space above the aircraft tank has fuel vapor mixing with air (at lower oxygen than normal thanks to the inerting system). During refueling, the fuel will replace the space used by this fuel-air mixture and force the mixture out of the tank. If unmanaged, this is a big fuel source and fire hazard.
How is this mixture managed in order to minimize risk of fire or explosion?

Nice question, never thought about it. I learnt something.

In CA at ye auto gas stations nozzles are required to have some kind of fume vacuum hood system for this reason i presume. Does that tech get used on avgas nozzles or anything?

Large airplanes are pressure refuelled so vapor escaping through the fuel port is not an issue as it is in smaller airplanes.  You are correct, however, that the fuel/air mixture existing in the tank will be displaced by the incoming fuel.  The wings will have vents installed that will allow the air mixture in the tank to escape (and fuel in the case of over fuelling or excess expansion of the fuel after it is loaded).
The fire risk is minimized in three ways I can think of:

The airplane is grounded to the fuel truck, minimizing the risk of ignition via sparking.
The wing vents have flame arrestors so that if the air mixture leaving the wing did ignite, the flame would not make it into the tank.
Atmospheric dispersion -- once the air mixture is out of the tank it will become more dilute with time.

In the cases there is more than vapor released, e.g. liquid fuel is expelled, these spills will be absorbed to minimize fire risk.  In the ultimate case of fire the fuel truck has fire extinguishers and there is a fire station on the airport that can apply foam and water as needed.

and of course large open spaces like airport ramps are typically windy, speeding up dispersal quite a bit. Hence the rules to not refuel inside hangars, but tow the aircraft outside...


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Is on-board equipment for in-flight de-icing also used on the ground?
Aircraft operating in icy conditions are equipped with de-icing equipment to remove ice from specific parts of the aircraft (wing, sensors,...) in flight.
When there is snow or icy conditions while the aircraft is still on ground, it is de-iced with ground equipment (de-ice fluid, towel,...).
Is the aircraft's own de-icing equipment also used?
Is this equipment enough to de-ice an aircraft before take-off (lets say if this is a sunny day and the temperature is just below 0°C)?

Why would you want to use the plane's very limited resources to do something that the airport is better equipped to do for you?

@DavidRicherby To keep generality, I never specify the aircraft type neither the airport. It may be a military transport aircraft transporting stuff to (re)build the airport it is standing on. It may also be a A380 standing at LAX, a DHC-6 at Lukla, ... And what I want is knowledge about how it is done, possibly with references.

@DavidRicherby Because the airport you're at isn't equipped to do it. So, start up the airplane, turn the prop heat on, pop the boots a few times, then shut down and get busy with a whisk broom, rags, and whatever else will work.

Usually, no.
There are two main methods used to deice an airplane in the air. One is letting warm air that is bleed from the jet engine to heat the wing leading edge. The other is to have have rubber air bladders (called deice boots) on the leading edge of the wing and tail surfaces that are pumped up and the ice falls off. Both of those methods are only useful to deice the leading edge of the wing and tail surfaces which is pretty much the only dangerous place where an airplane can gather ice while it is flying.
Airplanes on the ground are a different story. Ice can be built up pretty much anywhere as there is no airflow to prevent it. Excessive ice on control surfaces can make them behave "unevenly" or in extreme cases even unresponsive. Bigger chunks of ice breaking off the fuselage can get ingested by the engine and cause problems. It is therefore important that the airplane takes off ice-free. In fact, as little as frost on the surface of the wings can cause a significant loss of lift and can even cause a plane to crash.

You begin your answer with "usually". Does it mean there are cases where you use both aircraft and ground de-ice capabilities at the same time?

@ManuH It is not improbable that small GA airplanes that are fitted with technologies like the TKS Ice Protection system or passive anti-ice materials are theoretically able to adequately de-ice themselves on the ground.

@RRR Most of the TKS systems I'm familiar with for light GA ("weeping wing" systems and prop slingers, like CAV Aerospace's system) require some forward movement of the aircraft to keep the wings clean (the fluid sheets back in the slipstream). Prop slingers would probably work at ground idle, but I'm not sure how effective they'd be at getting deice fluid to the tips...

@RRR I agree with your statement that the leading edge of the wing is pretty much the only dangerous place where an airplane can gather ice while it is flying if the aircraft is a jet. However, propeller icing can also be dangerous. Antennae icing, while not dangerous, can also be a significant nuisance, even to the extent of an antennae tearing off (happened to me only once). Also, again in the nuisance category, ice on 747 windshield wipers can cause them to vibrate and set up a loud buzz in the cockpit that is very annoying.

Yes, anti-ice equipment is sometimes used on the ground, but is usually not sufficient to clear the plane of ice.
Airplane systems are generally referred to as anti-ice, because they are designed to prevent ice from forming or accumulating. Airport equipment is referred to as de-ice, because it is designed to remove ice as well as preventing it from forming (to some degree). So, as others have said, the real de-icing equipment available on the ground is much more effective than what is on the plane.
Anti-ice systems are generally designed to protect key areas of the airplane where ice is most likely to form in flight; leading edges of the wings, engine cowls/props, and pitot-static or other sensors. However, on the ground, it will accumulate on other exposed surfaces like wings/stabilizers/fuselage. This is why the de-ice measures are needed, to clear off those surfaces as well, because ice on the wings is bad.
However, anti-ice equipment may be used on the ground as well when needed. Air Florida Flight 90 crashed shortly after takeoff in icing conditions, and a contributing factor was ice accumulating on engine sensors which prevented the engines from providing full thrust. So when icing is expected, the anti-ice systems will certainly be used on the ground to provide added protecting to the most critical surfaces, in addition to the protection from de-icing procedures. Failing to have the wing and control surfaces properly de-iced also contributed to that accident.
Also, see this related question: Why turn off pitot tube heating?

Good points. We used engine A/ice on the ground in visible moisture with SAT <= 10C to prevent similar issues to that air florida crash.

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How much was the engine moved in the B737 Max w.r.t. older versions?
In the wake of the sad crash of ET-302, I am reading on multiple sources that

the 737 Max's "engine is a bit further forward and a bit higher in relation to the wing, compared to the previous version of the plane. That affects the balance of the plane"

(from the BBC)
Is there data available on how much these "bit further forward" and "bit higher" amount to?
In particular, to minimize misunderstandings, it would be ideal to know how much the CoG of the engines has moved w.r.t. the neutral point.

Without a better answer, you could try and estimate it using the scale drawings from the public Airplane Characteristics for Airport Planning manuals: https://www.boeing.com/commercial/airports/plan_manuals.page

Comparing the -800NG (https://www.boeing.com/resources/boeingdotcom/commercial/airports/acaps/737.pdf page 39) and Max 8(https://www.boeing.com/resources/boeingdotcom/commercial/airports/acaps/737MAX_RevC.pdf page 2-11), does not seem to be any significant change.

@DeepSpace I wouldn't know how to determine that from those figures, since the numbers are given w.r.t. something that changed for sure (the fuselage is longer, so the nose is further ahead) and the outlines are not required to be exactly to scale, I think.

@Federico There are scale drawings at the very end of the manuals...

@CptReynolds not really reliable if we are trying to determine changes in the centimetre range, right?

@Federico Of course. So... what exactly are you after? What effects would 15cm up have as opposed to 25cm up?

@CptReynolds I'm after the exact numbers, as stated in the question, not "a bit" or "about x cm".

@Federico I understand. I just wanted to also understand what the exact numbers would mean.

The Boeing 737-800 and 737 MAX 8 both have the exact same fuselage length of $ 39.47 \, \mathrm{m} $, so I chose these two models for comparison.
Boeing publishes CAD drawings for all aircraft models. I used the two CAD files for the 800 and MAX 8 to compare the engine size and location:

Since I do not know the units used in the CAD files, I used the known total fuselage length to scale the numbers:

Distance from the nose to the front of the engine nacelle:

$$ \text{737-800:} \; 13.544 \, \mathrm{m} \; , \; \; \text{737 MAX 8:} \; 13.180 \, \mathrm{m} $$

Length of the engine nacelle including exhaust:

$$ \text{737-800:} \; 5.075 \, \mathrm{m} \; , \; \; \text{737 MAX 8:} \; 5.345 \, \mathrm{m} $$

Diameter of the engine nacelle:

$$ \text{737-800:} \; 1.978 \, \mathrm{m} \; , \; \; \text{737 MAX 8:} \; 2.407 \, \mathrm{m} $$

Distance from the top of the engine nacelle to the bottom of the fuselage:

$$ \text{737-800:} \; 1.359 \, \mathrm{m} \; , \; \; \text{737 MAX 8:} \; 1.554 \, \mathrm{m} $$
Based on these measurements, the new engine nacelles are $ 270 \, \mathrm{mm} $ longer and have a $ 429 \, \mathrm{mm} $ larger diameter. They start $ 364 \, \mathrm{mm} $ further forwards and the top is $ 195 \, \mathrm{mm} $ higher.
Therefore, the center (geometric, not necessarily center of gravity) of the engines has moved $ 229 \, \mathrm{mm} $ forwards and $ 19 \, \mathrm{mm} $ downwards. While the geometric center may have moved downwards, the center of gravity probably moved upwards because the old nacelle was not symmetric (flattened at the bottom).

The dimensions given are in inches.

From the drawings it looks like the radome is a bit more elongated in the MAX version, as well as the tail cone. I'll accept these numbers for the moment anyway.

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How to find altimeter settings for higher altitudes?
I am trying to calculate cruise performance, but how would I find the altimeter setting for a given altitude?
I’ve looked at charts but they only show altimeter settings for the surface.
Would I use the alt settings at the surface and subtract an inch per thousand feet, or does this only apply in std conditions and if it does how would I find that altimeter setting so that I can calculate cruise performance..

There's no such thing as an "altimeter setting for a given altitude." You use the same altimeter setting all the way up to the transition altitude (18000 feet in the US, but it depends where in the world you are).
Performance charts are usually given in terms of pressure or density altitudes. You can use the altimeter setting and your intended cruise altitude to compute a pressure altitude and temperatures aloft to compute a density altitude from there if needed. Then use this to compute performance.
Your cruise altitude is measured in terms of indicated altitude, so if you are cruising at 4500 feet with an altimeter setting of 29.92 inches of mercury, your pressure altitude will always be 4500 feet (ignoring instrumentation error). On a standard day, your true altitude will also be 4500 feet, while on a non-standard day your true altitude may be considerably higher or lower than this.

Use the current altimeter setting at a surface airport within 100 NM of your location and that will be accurate enough for cruise calculations.

Here is the reasoning behind this.  The airplane is flying a pressure altitude of (for example) 8,500 FT.  It is not going to be the true altitude above mean sea level as the pressure can change based on many factors.  The airplane senses it is at a pressure altitude of 8,500 FT.
Since we are properly using the Kollsman window correctly and setting the current altimeter setting within 100 NM of your location, the difference between a pressure altitude of 8,500 FT and your altitude is the difference in the calculation for the altimeter setting.  At altimeter setting of 30.02 indicates you are flying a pressure altitude of 8,400 FT when the altimeter shows 8,500 FT.
The pressure altitude and the temperature at that altitude is what is required for the cruise calculations.

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What are the different structural weight limitations that have to be considered when loading large aircraft?
What are the different structural weight limitations that have to be considered when loading large aircraft? Is there a difference in complexity as to whether it's a passenger aircraft or a freighter being loaded?
I think I know the answers, but it's precisely because I have to say "I think" rather than just "I know" that I'm asking the question. I'm hoping those here more technically oriented than I will be able to add to what I know.

Perhaps we should have a Q&A here regarding those pesky "tall rigid cargo" requirements as well?

Hi Terry, I guess you wait for my input. Sorry, but I don't have much experience here. All I know is that the most severe design requirement for passenger floors is the stiletto heel, and the carpet becomes a structural member to avoid puncturing the composite sandwich floor panel. Freighters should be more complex once freight is not containerized, and here the items must be palletized to avoid exceeding the local floor pressure. The loadmaster must ensure that the load is properly distributed and tied down - after all, in contrast to the flight engineer, the load master is still required.

@PeterKämpf It's a tediously complex issue, at least in my opinion, and though I perhaps know more about it than most pilots, there are a lot of holes in my knowledge. I have tended to learn just enough to write software to do large cargo aircraft weight and balance, but there are significant gaps in my knowledge. I was hoping someone here might be knowledgeable in the area, but I'm not surprised that there have been no answers. In the next day or so I'll put up an answer giving what I know.

Great! Once I see your answer, maybe I can add what I know.

Is there a difference in complexity as to whether it's a passenger aircraft or a freighter being loaded?

Yes, very much so. The resulting load of a passenger in every seat is nowhere near close to the structural limits (note that I said structural limits, not performance limits). Generally speaking, having to check for the load being within structural limits is a concern for freighters, not passenger aircraft.
There are a lot of structural considerations for the semi-monocoque design used for all modern large aircraft. Using one variant of a 747-400F as an example, its Weight and Balance Control and Loading Manual runs to 584 pages. Items that need to be checked for each load include:
Linear and Floor Loading

The manual snippet above covers the main deck; there are like tables for each cargo area. The linear loading is the lb/in measured along the longitudinal axis of the airplane. I don't really understand why it's necessary to check both the linear loading and the floor loading, given above as lb/sq ft.
Not only does the linear loading for each cargo compartment need to be checked, but the combined linear loading of all decks together along the longitudinal axis has separate linear limits as tabled below:

Cumulative Loading
There are cumulative load limits for both the forward body and the aft body. Using the forward body for example, you add up individual items (more often than not ULDs) starting at the front and proceeding to a point over the wing box. I'm not sure exactly how that point is chosen insofar as structural and aerodynamic considerations are concerned. The aft body cumulative load starts from the tail and proceeds to the same point.
Below is a forward body cumulative limits graph. Note that there are two lines: one for use when the ZFW is 610,000 lbs or below, one from that weight up to 635,000 lbs. For this particular airplane, the aft body also has two lines, but the differentiation is based on the takeoff c.g.. Some 747s have only one line for each, some one line for one but two lines for the other. Obviously things can get tediously complex, and loadmasters and the software they're using has to accommodate all the possibilities.

Unsymmetrical Main Deck Limits
Widebody aircraft that can accommodate two rows of ULDs on the main deck have the option of carrying more weight in one container if the container laterally next it is given a lower limit. I know of one operator that didn't want to contend with the limit of, say, the position on the left being higher than the position on the right, and chose to forego the flexibility of unsymmetrical limits. An unsymmetrical limit graphs is below:

So, does anyone want to do a weight and balance for a large cargo aircraft? Fortunately we have software to do that for the great majority of loads that are containerized. However, loadmasters might still have to contend with unusual loads manually.

The staggered linear load limits are for limiting the fuselage bending moment. This can be critical both in max-g cases and in pitch motions, and the mass should be as close to the center of gravity as possible. The values are staggered to make compliance easier. This is also the reason for the unsymmetrical lateral limits: If one container is lighter than allowed, the one next to it may be heavier, so the total allowable moment around the center of gravity can be fully used. The floor loading is for local strength limits: Heavier loadings will overstress the floor panels and floor beams.

The increasing steepness of the limit lines in the Allowable Cumulative Load chart shows that the last lbs of payload added should proportionally contribute less lever arm. I think the X-axis actually shows moment, not length. Again, the lines are in linear steps to make calculation easier. Ideally, the limits would be smooth curves.

The staggering may also coincide with changes in the skin and stringer gauges. Normally, the fuselage weighs more per foot/meter around the middle than at the ends of the cylindrical section, because the bending loads are highest in the middle, and to keep stresses constant, thicker cross sections are required. Normally, gauge changes are staggered to make manufacturing easier.

It doesn't look so hard. Generally, it comes down to treating the aircraft gently and to distribute loads such that the mass centers around the center of gravity. If the mass of all items is known, planning the loading should take half an hour, and less with more experience.

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Why would an airliner make a 90° course change?
Why would this airliner be doing a 90° course change at cruising altitude? I saw this while waiting for a traffic light to change.
9 Jan 2020 1:42 pm (18:42 UTC) at Virginia Beach, VA, coordinates:
36°46'38.070"N 76°05'18.03"W, my heading: 237 deg.

And from Flightradar24, flying at 37,000 feet:

Related: What aircraft can make these sharp 90-degree turns?

For review purposes: These questions are not duplicate - the linked one refers to technical capability to make sharp turns, this one to the purpose, and it uses tracking data on the map.

This answer assumes the manouevring was related to the Military training activities going on off-shore. While plausible, there is another plausible explanition in ymb1's answer.

That was a British Airways Boeing 747 on its way from London to Miami.
Just off the coast there is a military exercise going on (see the LJ35 doing circles in the screenshot from flightradar24.com below; probably a military contractor like the one in this question).
Therefore all flights that would normally fly in that area end up flying over the coast. It seems that when the peak traffic from Europe towards Miami was passing, ATC decided to put everybody onto their original route as soon as they passed the bottleneck in order to reduce the traffic density. The result is quite a few sharp turns.

A few hours later there were at least 4 Learjets playing around off the coast. Note that most military traffic is not shown on FR24, so it may have been quite a bit busier.

Perhaps unrelated, but what are a bunch of Lears doing near a military exercise? Are those C21-A (Learjet 35A in military service) for observation purposes?

@Mast did you follow the link provided  in this answer? It is explaned there.

Ah, of-course. They're towing the targets. Thank you.

@DeltaLima, may I ask how do you show multiple aircraft track on Flighradar24 in the first screenshot please?

When you open fr24 in the browser on a desktop, there is a combobox in the top right corner where you can choose "Multi Select". I think the number of aircraft you can select depends on the type of subscription you have.


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How are AFTN costs charged to ANSP's and Airspace Users?
I'd like to know how ANSP's like the FAA/SENEAM/etc.. get charged for sending and receiving messages over AFTN. I'm wondering if it is on a per characher/message basis or if they pay in bulk. Also, how do private Airspace Users or 3rd parties like Jeppesen pay. Since this network is actual copper and is so old, I'm wondering if SITA makes pure profit or if it is more complex than I understand.

The short answer is it's much more complex than you think. Communications between ANSPs and users is not always through AFTN.


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What are the differences between ADS + INS and ADIRS / ADIRU?
What is the main difference and advantage in the Air Data System, between the former architecture (ADS + INS) and the newer one (ADIRS/ADIRU)?

The primary difference is packaging, which saves space and weight. The ARINC 704A IRU is a 4 MCU size LRU. Likewise, the ARINC 706 ADC is also a 4 MCU unit. The ARINC 738A ADIRS combines both functions into a single 4 MCU LRU. Though the ADIRS also includes remotely mounted Air Data Modules (ADM) which reduce the amount of pitot/static plumbing.
Functionally, the ADIRS performs the same functions as the separate IRS and ADS. A big drawback to the older ADS was the need to run the pitot and static lines all the way to the LRU. These lines are a maintenance headache, requiring multiple connections all of which have the potential to leak.
The ADMs can be mounted near the pitot and static sources significantly reducing the plumbing. The ADM converts the pressures to digital signals passed to the ADIRU via ARINC 429 or similar data bus. This simplifies installation, maintenance, and weighs less.
It should be noted that newer aircraft are moving to even newer architectures. The Boeing 787 dispensed with ADIRS and went with the Goodrich SmartProbes for air data and an Earth Reference System (ERS) to replace the IRU.
The Goodrich SmartProbes do away with pitot/static plumbing entirely by incorporating the pressure conversion in the probe base. They also electrically cross-connect the probes to perform the ADC function with no separate LRU.
The ERS addresses the FAA/EASA concerns over common mode faults in the IRU by using an IRU/AHRS mixed sensor architecture. In each of the two ERS, the IRU acts as the primary reference with the AHRS acting as a monitor. The key is that the IRU and AHRS use different hardware and software so that a common design flaw can't affect both and cause an undetected failure.
It's just evolution in the function of the aircraft sensors. They do what they need to do. The big demand from Boeing, Airbus, et al, is to reduce size, weight, power consumption, and cost.


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Are the guns on a fighter jet fixed or can they be aimed?
Are the (built-in) guns on fighter jets fixed, or can they be elevated up and down or panned right and left for aiming?
In other words are fighter jet guns aimed by pointing the whole aircraft at the target or can they be aimed independently?

They can be aimed in any direction, at high enough speeds, using the plane's control surfaces :)

And on the A-10, the gun is the airplane. But that's a ground attack airplane, not a fighter, though.

re Jörg's comment: Here's photo of the A-10's GAU-8 cannon... the aircraft is literally built around that thing

The use of thrust vectoring is quite relevant here, and I would think the following article confirms this. http://www.codeonemagazine.com/article.html?item_id=163  To quote"vectoring allowed the offensive pilot to get a quicker kill, and it helped eliminate some of the common mistakes of overshooting or getting stuck in lag (a situation in which the aircraft can't quite get its nose onto the bandit because of the AOA limiter)." I would also encourage you to try out an air combat simulator, even a simple one like YSFLIGHT to get a feel for air to air gunnery.

The guns of modern fighter jets are fixed to the airframe—they can't be moved in any direction; the pilot has to maneuver the aircraft to bring the guns to bear. The image shows the mounting of General Electric M61A1 Vulcan cannon in an F-16.

 Photo copyright (c) Meindert de Vreeze. Source: designer.home.xs4all
Gun installation in F-16.

The pilot aims the guns using an optical representation in the Heads-Up Display (HUD). Some of the aircraft use the Lead Computing Optical Sight where:

... a prediction angle is continually computed and displayed on the Heads Up Display (HUD) as a reticle offset from a fixed reference on the gunsight. ... (which) accounts for ballistic curvature, own motion, and predicted target motion.

Basically, it shows where the shells will pass through at a certain distance. Some aircraft use Enhanced Envelope Gun Sight (EEGS). It basically shows a pair of lines which the cannon shells will follow- the lines moves as the aircraft maneuvers, showing the effect of the maneuver on the flight path (of the shell). The correct 'solution' exists when the wingtips of the target touches the funnel lines.

EEGS; image from simhq.com

Excellent answer, thanks for the cutaway picture and details about aiming.

They are fixed, and many attempts were made over the years to have an aiming capability - and all failed. That should tell you something.
The earliest aereal combats used indeed handguns which were pointed at the adversary by the pilot. But it soon became obvious that only the rapid fire of a machine-gun will give the pilot a realistic chance of hitting a fast-moving target. To learn what has to be considered when aiming at aircraft I recommend to watch this B-29 gunnery instruction movie on YouTube.
During the Italo-Turkish war of 1911 and 1912, the Italians used aircraft in war for the first time. Since the Ottoman Empire had no air force yet, the Turks engaged European aviators who flew their own aircraft, taking a handgun along to shoot at the Italians.
The same was tried in the opening months of WW I, but it soon became obvious that only a machine gun was effective. Before gun synchronisation was invented, pusher designs were fielded, some of which carried a dedicated gunner in a seat ahead of the cockpit.

Vickers F.B.5 "Gun Bus" (picture source)
Of course, the much smaller and nimbler Fokker single-seaters with gun synchronisation made such designs soon obsolete.
When Great Britain prepared for WW II, the firing power of bomber turrets impressed the Generals so much that they ordered several dedicated fighters with a gun turret mounted right aft of the cockpit. The resulting Bouton-Paul Defiant was again too slow and easily outmanoeuvred by the single-seaters, and the Blackburn Roc and the Hawker Hotspur were even worse. The Defiant's only successful role was as an early night fighter; when attacked by day it could only join others in a Lufbery circle to defend against being shot down itself.

Bouton-Paul Defiant F. Mk.I (picture source). Note the retractable belly-mounted antenna masts which gave the gunner a free field of fire in the upper hemisphere.
Another attempt was made by the MiG design bureau in the 1950s. In a dogfight the pursuer needs to get inside the circle of its opponent in order to score a hit with a fixed machine gun. The Russians thought: Why not pivot the gun into the circle, so the pursuer can fly the same circle as its opponent and still shoot at it? A MiG-17 was modified accordingly, but flight testing soon revealed that the recoil from the machine gun would push the nose of the fighter down and outside of the circle when the gun was not aligned with the longitudinal axis of the fighter, so this idea was dropped very soon.
Now I need to mention that rarely are the guns completely aligned with the longitudinal axis; instead, they point slightly up to compensate for the ballistic bullet drop at the expected distance of the target. Also, wing-mounted guns point slightly inward, again to ensure the bullet path is straight ahead when it catches up with the target, a procedure called gun harmonisation. This also means that interceptors will have their guns pointing up slightly more than those of air-supremacy fighters. One particular case involved the MiG-15: Being designed to fight large B-29 formations, the guns were badly aligned for fighting the F-86 Sabre during the Korea War. The MiG-15 armament consisted of two 23mm guns and one 37mm gun with very different ballistics. In order to ensure that the path of all bullets would meet at the right distance for fighting bombers, the 23mm guns pointed more up, and when used against the much smaller F-86, the Russian and Chinese pilots opened fire at a shorter distance so that the 23mm bullets would fly over the target while the 37mm bullets went below it. This was a major reason for the poor kill ratio of the MiG-15 vs. the F-86 despite the MiG's better performance.

Gun harmonisation on a jacked-up P-47 (picture source)
Modern high-off-boresight missiles with helmet-mounted cueing (or sight-controlled missile targeting) have turned that longtime dream of shooting at what the pilot is looking into reality, and the gun is carried largely as a low-priority weapon of last resort.

ah, the ineptly named Defiant :) Mind of course that in aircraft back to WW2 guns also are often used to give the aircraft a ground attack capability. Handy when going after that landing Me-262 and shooting up the rest of his squadron on the ground before bombers can arrive to crater the runways.

Ah the modified Mig-17 addresses my question about a permanently offset gun. Very interesting.

@ToddWilcox: Yes, the Russians simply went ahead and tested all kind of weird stuff. That is the best teacher.

+1 for thoroughness, but in WWI non-fixed guns, such as the Lewis gun or the Bristol Fighter, were extremely effective: that is probably what led to the Defiant design.

Chuck Yeager, after test flying a MIG15, had a general ask him how bad the longitudinal oscillation motion was. Yeager replied - about right to hit a B36 wingtip to wingtip if you were shooting at it.

@tj1000: That should had been the lateral motion. Typical for high speed flight; the P-51 was even worse than that.

They are fixed. The pilot must maneuver the aircraft such that the trajectory of the bullets will impact his selected target.
Here is a picture of the HUD on a F/A-18 during a gun run on a ground target. Note the circular symbol which indicates where the bullets will hit.
(Screenshot of JetStream Episode 8)

Which circle are you talking about? I see two.

@pnovotnak the large circle with a dot in the middle.

I was a light attack pilot in the 1980s and flew off the USS Nimitz. I don't want to give anyone the idea that air-to-air warfare is something that can be scripted. It is highly complex, like chess, constantly changing, where deception and trickery often win the engagement over technology and a high performance airframe.
Consider this: A light single prop aircraft, maybe something like the Douglas A-1 Skyraider, can be a formidable threat when attacking a carrier with even a small bomb hung under the wing. It is slow with a tight turn radius, can fly very low, and has a very small heat signature. All these things can be used to its advantage.
I can add a few things of interest. The guns on the attack and fighter jets were fixed in angle, and aligned above boresight. I know this to be the case for the F-14. This was so that when you were tracking a target in a high-g turn the gun would be pointing ahead of the turn, instead of behind your adversary in the turn. On the other hand the A-7E had its cannon boresighted, because it was aligned for strafing.
Having a gun aligned above boresight can have its problems. I was on a war-at-sea strike against a destroyer and was intercepted by an F-14 at around 50 miles out. I was cruising the wave tops at around 650 knots. The F-14 stayed in a position above and behind and did not engage me as I flew in on my attack. Later we met back at the ship and he told me that, at such a low altitude, he would have to go inverted to get his forward pointing gun on me to get a kill. He said this was something he was not willing to do, and so he waited for a better opportunity. Perhaps I would pop up coming in to the target for my to delivery.
Which brings me to the more interesting consideration. The tactics of air combat maneuvering (ACM) have evolved over the years. When I began ACM training (1980) in the A-4 Skyhawk we were taught that you needed to gain a 6-o'clock position in the fight, and be tracking your target for at least six seconds to have a successful kill.
Later in the mid-1980s things had changed and we were being taught the snapshot. Israeli experience showed that most dogfights quickly degrade in to "furballs" with many-to-many engagements. If you track a target for six seconds in such an engagement you will be dead. Essentially, there is always someone finding themselves at your 6 o'clock. So we were taught to attempt to position yourself outside the furball and blow through the fight without engaging any particular target too long, taking shots of opportunity. These shots typically had to be taken with large angles and lots of closure. Picture someone crossing your canopy from left to right in a high-g turn. You would use "Kentucky windage" and take your shot so the opposing aircraft ended up flying through the 20 mm cannon shells you fired at 6000 rounds per minute. So much for "aiming."
One final comment. The F-4 Phantom was designed without an air-to-air cannon and carried only missiles. On the F-14's, for example these weapons were close in missiles, e.g. the AIM-9, and longer range (over the horizon), e.g. the Phoenix and Sparrow missiles. There was no gun on the F-4, because prior to Vietnam it was thought that aerial dog fights were a thing of the past with the advancing technology. This of course turned out to be a mistake, and gun pods were mounted on F-4's to give them a close-in capability, while at the same time sacrificing performance. The number of missiles carried on one aircraft are expended rapidly against a large number of targets, and then one is reduced to battling it out close-in.

'war-at-sea strike' in an A-1, presumably? 'go inverted' = fly upside-down? Did the F-14 not have missiles?

The A7E comprised the light attack aircraft aboard a carrier: 2 squadrons. They actively participated in war-at-sea strikes as exceptional VFR bombers. The tactic was low level ingress to the target, at 3 miles pull hard to 15 degrees nose high, roll inverted at around 2,500 feet AGL, look for the target, pull down hard on the target, deliver and egress as quickly as possible, with the rest of the air wing coming in after you with separations on target at 15-30 seconds. F14's have 4 AIM-9's and they figure they will be gone quick. Using an Aim-9 I would be dead.

Aaron: those WASEX's were a hoot.  Most of mine were in the Op Area not far from Vieques; on frigates and destroyers.  The A-7's attacks were a fun to watch from the deck of the ship as they came in.  Cool stuff you guys did.

Wish I could have been there. We did the whole Air Wing from time to time too. 40 aircraft coming in at 500 ft.

Paragraph 4 - why would F-14 not porpoise-up then come down to give a better line ?  Too close to the water and insufficient margin of error ?   Or a loss of airspeed make him fall behind ?

@Criggie I did ask the pilot that question. He said that he was not comfortable taking the aircraft into a dive that low to the ground. He expected me to do a pop-up delivery close to the target, and decided that would be where he took his shot. With him back there, and getting to the target, I would have done a lay down delivery instead.

The internal cannon on a jet fighter is s permanent fixture in the aircraft, hard mounted to the airframe and cannot be swiveled independent of the flight path.
To the second question, yes, aiming the gun involves pointing the aircraft's flight vector at the target or at a lead impact point in front of the target.  The pilot, in essence is riding a very large flying gun and many of the same shooting techniques i.e. breathing work, trigger press, leading are very similar to that found while handling and discharging a firearm, particularly a shotgun, at a moving target.
As mentioned above firing symbology and sighting arrangements vary from plane to plane.  Some common ones are the Lead Computed Optical Sight (LCOS) or Enhanced Envelope Gun Sight (EEGS) but all use either pre ranged hit points or a lead computed pipper - 'death dot' in fighter jock speak - with ranging cues.
It is possible, either now or in the near future, with position tracking helmet mounted cueing systems that flight control software can be developed for air to air use which allows the pilot to point the nose of the jet - and the gun - simply by looking at it.  The technology already exists to slave FCRs, IRSTs, gimballed FLIRs and weapon seeker heads to the HMCS, allowing for use of both radar guided missiles high off-bore sight IR air to air missiles as well as with ground munitions based on simply looking, designating and shooting.

Fixed wing - No, Rotary Wing - yes.  As they have previously stated the guns are attached to the airframe.  They do have revolving barrels, lots of rounds (that they go through at a very fast rate) but everything is aimed by the pilot.  We are good, but not that good yet.  It will be interesting looking aircraft when we can do that though.  Interesting question.
About the helmets in the F-15, 16, and 18.  That could be interesting as well.  However, currently in the helo community they are used to point say the M197 gun at the target.  I think in fixed wing, they may be used to point the aircraft.
This still proves what has already been posted though.  There is no way to point the weapon system without pointing the aircraft for fixed wing.

What is "Rotary Wing"?

@PeterMortensen A “Rotary Wing” is usually a helicopter (plus everything that has “gyro” in its name, minus the Greek dish).

The Saab JA 37 Viggen fighter jet actually had a system (automatsiktning) that would aim the entire aircraft instead of letting the pilot do it. It would make small adjustments to yaw etc. to make the cannon hit. (the cannon was still fixed though)
It was mainly used in frontal attack or long range rear attacks. It was very successful in frontal attacks.
The pilot would engage with radar lock and the computer would control pitch and jaw. The pilot controlled roll to keep the plane flying clean.
https://saabaircraftindustry.com/sv/vagar-till-ny-formaga/utvecklingskompetens/formageutveckling-av-styrsystem/#avsnitt-2074
https://www.fht.nu/bilder/Flygvapnet/Flygvapennytt/fv_nytt_2_93_j37_automatsiktning.pdf

There were at least MiG-15SU and MiG-17SP experimental fighters with a moving gun carriage V-1-25-Sh-3 with 2 Sh-3-23 guns mounted on it. Pilot could remotely adjust guns in combat (11 grad up or 7 - down), thus reducing time needed to get in firing position against maneuvering enemy. The system had no guidance of course.
Although I have no idea if this device was ever put in production, I believe in later aircraft designs stakes were made on missiles as primary weapon, leaving guns secondary role.

Guns are fixed in all modern fighter aircraft, and can only be aimed by aiming the entire aircraft. This is normally done with the aid of a Lead Computing Optical Sight (LCOSS). There are three components to an air-to-air gun solution, that must be taken into consideration by the engineers designing the gunsight mechanics, and well understood by the pilot utilizing the system.

Lead for Target Motion or Velocity Lead (Lm). This is the same thing as is utilized by a skeet shooter. You must aim the barrel of the gun where the target will be when the bullets get there, not where it is now. You cannot just point the gun at the target. You must point it some angle in front of the target in the direction the target is moving in your field of view. Lm always lies in the target's plane of motion. This requires that the system know the time of flight (TOF) of the bullets, and the Line-of-sight rate (LOS) of the target (how fast is it drifting across the inertial background in your field of view. If the target is moving at 20 degrees per second, and, based on the range, the TOF is 1.5 seconds, then you must aim 30 degrees in front pf the target, along it's projected velocity vector. Fighter pilots use a radar lock on to determine the range and TOF, and gunsights assume a Default range(in the F-4 it was 1000 feet) when there is no radar lock. The target LOS is determined by the gunsight by assuming that if the pilot is "tracking" the target, the shooter aircraft turn rate must be identical to the target's LOS. When the pilot is not tracking the target (as in a highly dynamic close range, high angle shot (called a snap shot), the pilot must understand this effect in order to turn on the gun with an additional lead angle to compensate for the target's relative motion in the windscreen. Effectively he pulls the trigger when the target, moving towards the gunsight aiming reference, appears to be about to cross that aiming reference about a second or two in the future.

Lead for Target Acceleration or Acceleration Lead (La). If the target was at zero Gs, (in ballistic free fall), there would be no need for La. Only Lead for target motion and Drag shift would be required.  La is required when the target is turning away from the ballistic path that the gunsight would be predicting without La. La always lies in the target's plane of acceleration (the plane the target aircraft is accelerating in (normally defined by where the lift vector or rudder points). The gunsight assumes that the target Plane of motion is the same as the shooter plane of motion, and that the the target G-load is the same as the shooter G-Load. The solution is based on the assumption that the target is accelerating or pulling Gs towards the shooter, and that the shooter, to match the target's turn rate, is pulling the same amount of Gs in the same direction. This is normally close to accurate when the target is at low angle off relative to the shooter, but is not accurate at higher angle off.

The last component is the correction for what is called Drag Shift. Bullets slow down after they leave the muzzle. This slowing causes the bullets to drift aft of their original line of fire (Line of departure or LOD). Imagine the top turret on a B-17 firing forward and upwards at a head-on approaching target. If the bullets maintained a constant velocity after they left the muzzle, all that would be necessary s to point the guns where the target will be one TOF from trigger squeeze (La), and correct for 1 G target acceleration (La). But the bullets slow down. That means it will take a slight bit longer to get out there, and the target will drift further aft in that small amount of time. Therefore, the gunner must aim slightly lower in elevation (less lead), to compensate for that. Imagine a high pressure water hose, forwards and upwards out of the sunroof of your SUV at 80 mph. The stream of water will appear to curve and bend backwards relative to the vehicle, the further it is from the nozzle. If you were trying to hit a drone flying 15 feet above you and 15 feet in front of you, you would have to aim in front of him to compensate for the apparent rearwards drift of the stream of water. This correction is required whenever the gun firing line is not aligned with the shooter aircraft velocity vector through the air. For a fighter, since guns are must be in lead, this means it reduces the the lead required by a small amount in the plane of target motion.

The relative contribution to the  total gunsight solution from these three components, generally, is as follows:

Lead for target motion:        70-85%

Lead for target acceleration:  10-25%

Drag Shift:                    5-10%

Pilots can aim by looking at their targets-even in High G situations.

These days, fighter pilots' helmets are nearly as complex as their
  airplanes. And with good reason: If the helmets can't tell where the
  pilot is looking, many of the airplane's systems, including weapon
  targeting, are useless.
The folks at San Jose, California, who have designed advanced U.S.
  military helmets for the F-15, F-16, and F/A-18, make their living at
  the nexus between man and machine. VSI's flagship program is the Joint
  Helmet Mounted Cueing System. It uses a magnetic field in the cockpit
  to sense the orientation of the helmet, then feeds information on the
  current line-of-sight to the aircraft's flight computer. VSI's helmet
  has an accuracy of about four milliradians, an angular measure
  commonly used in the world of shooting and targeting. One milliradian
  equates to one one-thousandth the distance to the target. So if the
  target is 1,000 feet away, you'd be accurate to within a foot.
Determining where a pilot is looking by tracking eye movement is a
  much taller order. "You would not believe some of the human factor
  issues you have to overcome to have a successful eye tracker," says
  Louis Taddeo, VSI marketing director. "Although we have research
  projects into eye tracking, it is a very difficult task both from a
  technical [standpoint] and the physiology."
So for now, VSI uses helmet position to achieve that four-milliradian
  accuracy. Pilots need only turn their heads to aim their weapons, even
  during high-G maneuvers, freeing their hands for other tasks.

Read more.

This system works for missiles but not the guns on fighter jets, which is what the question is asking about.

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review

@vsz I didn't copy/paste anything.  That is a recommended deletion from the review queue where I did indeed flag as "does not provide an answer".  The auto comment is exactly right.  This does not provide an answer to the question.  It further suggests that Rob requests clarification from the OP or, leaves a comment for them to understand what question is actually being asked.  I'm curious to understand why you think I used copy/paste and in what way the review queue comment is not appropriate.  If indeed you think it's wrong, I suggest a meta question.

@Simon : Ok, I understand. It was the second sentence which was odd.


text:
Categories of aircraft (A,B,C,D): Is it possible for an airplane to move into a different category?
Categories are defined for the Vat (Speed at threshold) which is Vso multiplied by 1.3, or Vsldg multiplied by 1.23 .
Since these speeds are related to the weight of the aircraft is it possible for an aircraft to start the flight in one category but then move into another one?

The FAA's Instrument Procedures Handbook, Chapter 4 has a good explanation of this (emphasis mine):

An airplane is certified in only one approach category, and although a
  faster approach may require higher category minimums to be used, an
  airplane cannot be flown to the minimums of a slower approach
  category. The certified approach category is permanent and independent
  of the changing conditions of day-to-day operations.

And:

A pilot must use the minima corresponding to the category determined
  during certification or higher. Helicopters may use Category A minima.
  If it is necessary to operate at a speed in excess of the upper limit
  of the speed range for an aircraft’s category, the minimums for the
  higher category must be used. For example, an airplane that fits into
  Category B, but is circling to land at a speed of 145 knots, must use
  the approach Category D minimums. As an additional example, a Category
  A aircraft that is operating at 130 knots on a straight-in approach
  must use the approach Category C minimums.

In other words, the aircraft's "certified approach category" never changes. However, for operational reasons a pilot may use a higher category's minimums for an approach, but never a lower category's. One possible scenario would be if the flaps are inoperative and the pilot can't slow the aircraft to its normal approach speed.

Does anyone know how the certificated approach category would be affected by variable maximum takeoff weight? Presumably the certificated approach category is based off maximum takeoff weight, but could it be reduced for one day if the maximum placard weight was reduced by engineering on that day?

Here is the wording (emphasis mine) adopted by ICAO (Doc 8168, Vol1, Section4, 1.3 - Categories of aircraft):

The criterion taken into consideration for the classification of aeroplanes by categories is the indicated airspeed at threshold (Vat), which is equal to the stall speed Vso multiplied by 1.3, or stall speed Vs1g multiplied by 1.23 in the landing configuration at the maximum certificated landing mass. If both Vso and Vs1g are available, the higher resulting Vat shall be applied.

So the category of a given aircraft doesn't depend on its current weight but on its maximum certificated landing mass, which is constant.


text:
What is the maximum rate of descent in an instrument approach?
What is the maximum rate of descent approved in an instrument approach procedure non-precision? Basically what is the maximum rate of descent (Fpm) that I use during the approach?

ICAO Doc 8168 stipulates that for a non-CDFA non-precision approach, the aircraft should not exceed 15% gradient when descending from FAF to MDA.

1.7.4 Stepdown descent
  The third technique involves an expeditious descent and is described as “descend immediately to not below the
  minimum stepdown fix altitude/height or MDA/H, as appropriate”. This technique is acceptable as long as the achieved
  descent gradient remains less than 15 per cent and the missed approach is initiated at or before the MAPt. Careful
  attention to altitude control is required with this technique due to the high rates of descent before reaching the MDA/H
  and, thereafter, because of the increased time of exposure to obstacles at the minimum descent altitude.

There cannot possibly be a maximum descent rate because the rate depends on the ground speed.
Instrument approaches are based on a fixed glide path with a typical slope of $ 3^\circ $ (but can be higher). Your descent rate will depend on the ground speed during the approach. E.g. flying at $ 150 \, \mathrm{kt} $ ground speed down a $ 3^\circ $ glide slope will result in
$$
\tan(3^\circ) \cdot 150 \, \mathrm{kt} \approx 796 \frac{\mathrm{ft}}{\mathrm{min}}
$$
Non-precision approaches may not have any vertical guidance available, but the chart should still specify the gradient and target altitudes at various points. When flying this approach at a given true airspeed, the descent rate will still vary with the wind because the gradient is fixed with respect to the ground, e.g. adding $ 10 \, \mathrm{kt} $ of headwind to the example above will result in
$$
\tan(3^\circ) \cdot 140 \, \mathrm{kt} \approx 743 \frac{\mathrm{ft}}{\mathrm{min}}
$$
The maximum descent rate will therefore depend on the airspeed you want to fly at during approach, the glide slope angle and the wind speed.

Actually, the angle of the approach can be from 3 to as much as 5.5 degrees, depending on the airport, but individual aircraft must be certified to be able to use the steeper approaches.

Lugano (LSZA) even has 6.65 degrees, so coming in at 100 kt ground speed (more realistic for a small airport) would result in 1181 ft/min.

Wow. That's steep! I wonder if there are any other published approaches that are steeper than that...

One of the approaches into Aspen (KASE) is 9.61 degrees, and the other two are just under Lugano.  Granted, it is technically a circling approach (VOR DME-C), but aircraft normally land straight in.

By the way, this is good information on a target rate of descent, but I don't think that it really answers the question of what the maximum descent permitted is.

The question does not have an answer for the reasons stated in this answer. There is no maximum rate of decent. In the USA the maximum would only be a rate considered safe and in control for the equipment, crew training, and conditions. Decent angles are fixed to the ground and not aircraft vertical or horizontal airspeeds or relative airflow angle.

Basically you should not exceed 1000 feet per minute after you have passed the initial approach fix. The actual rate of descent will vary with speed, the approach angle and type of approach as well as met conditions. Please see this article.

There is no "approved" legal maximum.  As a general rule of thumb, 500 fpm is the target rate you should use for IFR descents on non-precision approaches.

500 fpm corresponds to 3° glide-slope at 100 knots over ground. Higher speed or steeper approach need higher rate of descent.

Which at the typical approach speed of a typical light twin used in training is about where you want to be unless you are diving because you started late.  In a high perf aircraft it is closer to 7-800 fpm.   500 fpm is also roughly the limit for passenger comfort in unpressurized aircraft (and is the max rate that pressurization systems use for the same reason).

The standard descent angle is three degrees.  Assuming no wind, your sink rate is given by airspeed * sin 3 degrees, and it works out to about 5 ft/min/knot.  For a Cessna on final at 60 knots, the sink rate will be about 300 ft/min.  For a 737 on final at 150 knots or so (I'm used to seeing Vref30 + 5 = 148 knots in the simulator), you're looking at about 750 ft/min.
With wind, you'll have to calculate your along-track groundspeed component, and use tan 3 degrees instead of sin 3 degrees.  However, tan(3 degrees) and sin(3 degrees) are just about equal, and more than close enough for this particular flavor of government work.
According to a convenient calculator, sin 3 degrees = 0.0523, and tan 3 degrees = 0.0524.  That's close enough...
Incidentally, you CAN work this all the way through on an E6B computer.

In the USA the maximum rate would only be that considered safe and in control for the equipment, crew training, and conditions. Decent angles are fixed to the ground and not aircraft vertical or horizontal airspeeds or relative airflow angle.
Each segment of an approach has different slope and clearance requirements as they expect different aircraft configuration and technique in addition to local hazards(including odd wind patterns) and political rubbish like noise limitations. The details of a modern approach are calculated by computer due to the number of variables and the details of the formulas take up at least three 500 page FAA orders.(8260.3D, 8260.19H, TI8200.52, along with several other miscellaneous rules.)
There are maximum design descent angles but these depend on the approach segment, approach type, and aircraft category. For example a non-precision final approach segment for straight in landings with category A aircraft at 80knots or less can be up to 6.4degrees, while the slope is limited to 3.5 degrees for approaches that allow category D aircraft. The minimum decent angle is 2.75 degrees and 3.0 is preferred, but the angle should also be within 0.2 degrees of the visual slope indicator (ie. papi, vasi)(FAA order 8260.3D section 2-6)

If the Descent Gradient exceeds 400 ft per nm, the approach will be designated a circling approach, even if the course is straight-in.

-1 for pasting a screenshot.

Please add the actual text (which is then searchable) instead of the screenshot (which is not searchable). Also, please post the link to what you screenshotted, so that those who are interested can go to that page if they're interested in reading more.

Also, the question is asking about descent RATE (i.e. feet per minute) rather than descent GRADIENT (i.e. feet per nm). This post addresses the latter, but not the former.

Hi and welcome to aviation.SE. You may take a tour in the help center, especially the page named how do I write a good  answer? to understand why your answer is not well received and how you can improve it.


text:
Does a pilot follow the PAPI lights in the same way in all categories of aircraft?
Since there are different dimensions of aircraft in various approach categories, I wonder what changes pilots in different aircraft have to make when following the PAPI lights. I know that 3° (for example) for one aircraft is the same for all other aircraft, but different categories might have different touchdown points when following the same glideslope due to the MEHT.
Since the PAPI is fixed, what differences are then when flying it in two different aircraft? Does the size of the aircraft change the flight path in any way? Will I always touch down at the same spot?

What is MEHT???

@vasin1987 "Minimum Eye Height over Threshold"

The lights themselves are fixed, there is no way ATC or the airport operator can simply adjust them to suit the next inbound aircraft. If the pilot needs to they can fly the approach with 3 whites/1 red, but AFAIK that isnt really necessary in most situations (maybe for 747s and other longer aircraft).
It's worth remembering that the PAPI cannot provide precision to the foot. Having 2 white lights only means you are anywhere between about 2.8 to 3.2 degrees on a standard approach, which can be a considerable range when you're a few miles out.

Thank's for the reply @Ben. I was more concerned for the operational point of view though. I'll try to explaine me better; since the PAPI is fixed what difference can I note flying it with two different aircrafts? Does the size of the aircarft change the path in anyway? Will I touch at the same spot anyway?

To expand a bit on @Ben's answer and considering your comments.
All airplanes generally fly roughly the same final approach slope ~3° at most airports and all aircraft will follow the PAPI lights if they are there. Depending on the size of the aircraft, its current load and the environment, the aircraft may vary its speed and pitch to fly this approach correctly. For example a 747 will come into a runway way faster than my Piper Cherokee but will fly the same approach path (more or less).
PAPI generally provides ~3° approach path but more specifically it provides obstacle clearance and a safe approach slope and if there is an ILS system the lighting will coincide with the approach system. Although some airports can have considerably steeper approaches.

General siting considerations. The PAPI must be sited and aimed so that it defines an approach path with adequate clearance over
  obstacles and a minimum threshold crossing height. If the runway has
  an electronic landing system glide slope already established, the PAPI
  is installed as described in paragraph 502 so that the visual glide
  path angle will coincide with the electronic glide slope.

Flying at or above the PAPI lights will guarantee a safe approach and obstacle clearance in the area. The ILS and or PAPI systems take you to the Touch Down Zone which is where most aircraft aim on landing and where most will touch down in a regular approach. However for short runways (mostly flown in and out of my small GA aircraft) pilots may execute a short-field approach and landing which will attempt to put the aircraft down as close to the threshold as possible. In this case they may follow the PAPI close to the runway but execute a slightly different procedure when touching down.

I fly VFR in a light sport aircraft in an airport with a PAPI system. Because the airplane that I fly has a steeper glide slope than larger aircraft, I may start out high on the PAPI, and then get low on the PAPI, all the while descending in a normal (for the airplane) approach. Also, I've been told that the PAPI at my airport is not only set up for the 3 degree glide slope, but also to aim for the landing flare at the 1000' lines. I can usually flare and land before that spot, adding to the inaccuracy in the PAPI.
When I first started flying, I would frequently get nervous when I saw three red on the PAPI. Then I discussed it with my CFI and he explained to me how flying the PAPI glide slope is not going to be as useful for me as it would be for other aircraft.


text:
Why are flaps retracted when an aircraft is parked on ground?
I found this question. One of the answer tell the flaps are generally retracted after landing. I assume this mean flaps are retracted when the airplane is on the ground between flights.
Flaps are extended for landing and then extended again for take off. Not taking care of their position while on ground (no need of them) and putting them in the required position before take off may save (few) flaps actions (something like putting them from landing to take-off position instead of landing position to fully retracted and then from retracted to take-off position).
Is there some good reason to retract the flaps on ground? Does this good reason varies from a type of aircraft to another?

flap positions on for landing and takeoff are usually different.

On very light aircraft, the extra flaps might increase lift enough to put excessive strain on the tie-down straps?

I would bet it is for storage purposes. Flaps are more likely to get damaged by some knucklehead walking into them when they are down. Also, it might be because lowered flaps create openings that insects like to get inside and build nests in. Additionally, what raptor said about tie-downs is likely a reason.

According to many pilots, the flaps should be retracted on touchdown, or at least before taxiing, especially on windy days, to prevent the aircraft from becoming airborne again.

@raptortech97 not just very light aircraft. Few years ago there was a bad storm at Schiphol and they had to turn many aircraft so the wind wouldn't blow them over. Now imagine if those had had flaps extended, the wind was high enough that it would exceed the takeoff speed of a Fokker 50 with full takeoff flaps.

@ratchetfreak and that's why I said "putting them from landing to take-off position"

A few different reasons:

Good pilots put the aircraft into a well-known (up) configuration after landing, so that the aircraft is ready for use on the next flight.
Flaps down during taxi was a signal to the tower that the aircraft had been hijacked.
Take-off flaps (10 degrees on my airplane) and landing flaps (45 degrees), are nearly always different, so leaving the flaps down after landing at, say, 45, doesn't save any steps for departure. If the pilots forgets to set flaps before takeoff, some aircraft (like mine) will make it almost impossible to depart and climb out on full flaps because of the induced drag, while takeoff with no flaps is possible.
Prevent trucks and people from hitting the flaps while the airplane is parked, especially in the full-down (landing) configuration.
Some pilots employ the practice of retracting the flaps during the rollout because it puts more weight on the wheels, reducing stopping distance.
Flaps down increases the amount of lift generated by the wing, so some aircraft might actually lift off the ground while parked with full flaps in high winds.

for the reason #3: the sequence 45 degrees --> 0 --> 10 is one step more than 45-->10. And I hope any pilot will check the flaps positions before take off and thus not take off with full flaps. the reasons #2 and #4 seems enough to take care of flaps position

All of these reasons contribute, but but the first answer is the most correct. Good pilots follow checklists, as per the PPL PTS -- "Throughout the practical test, the applicant is evaluated on the use of an approved manufacturer’s checklist or equivalent" -- because checklists reduce accidents.

Do you have any documentation on #2 ? I'm curious about that :)

@QuentinHayot i'm glad you asked!  http://fsims.faa.gov/wdocs/orders/ps_orders/a_7110.49d.htm  6(3)(b) "PILOT SIGNAL (Covert):  Leave full flaps down
after landing or lower full flaps while on the ground."

You never know what wind speed will hit the aircraft until the crew returns. Also, retracted flaps are much less of an obstacle to careless drivers of airport trucks. This video shows a parked 747 in high winds. Now imagine what would have happened if the flaps had been deployed!
Note that fully extended high lift systems at ground angle of attack can produce five or six times the lift of the wing with flaps retracted. Lowering the trailing edge flaps increases wing area and lowers the zero-lift angle of attack, so at ground attitude the wing is much farther up on the lift curve slope.
In aircraft with manual controls it is also recommended to secure all control surfaces, so they don't float in the wind. Bearings and pushrods could be damaged, and in a careless preflight check might not raise suspicions, but can go on to fail in flight. Better be safe and reduce all chances of wind playing with the aircraft!

The linked video shows a partially dismantled aircraft, so it may not be representative. But there are also accidents like this ATR72 in Shannon

Some comments on the video in question suggest that flaps were deployed at the time. I can't tell at all, though.

The video's description says the flaps were deployed, but also that the engines were removed, so the center of gravity is further aft than normal. @raptortech97

@raptortech97 That appears to correct. If you look at  flap track fairings, their angle shows that the flaps are deployed.

On larger aircraft it would be standard operating procedure to retract the flaps after landing. This is to ensure the crew taking over from you are presented with an aircraft in the correct state in accordance with company SOPs.
Flaps could easily be damaged by ground crew loading and unloading. Also it would be difficult to refuel the aircraft with the flaps/slats extended (Larger Boeing types have the refuel system located at the leading edge underneath a wing).
There are occasions when you would leave the flaps extended, during operations in heavy snow for example - retracting contaminated surfaces may cause damage.

and then the obvious one... you retract your flaps so you don't keep hitting your head on the flap when you are bent over walking around your plane  :)

Only on a high-wing plane. I did most of my flight training on a DR-400, where you stand on the wing to get in and out of the plane. Before getting out of the plane, you must extend the flaps in order to prevent people from walking on them.

On a PA28 there's no danger of walking into the flaps, but if you have them extended and try to step on them getting out of the plane they'll continue extending and drop you on the ground :)

On Pipers (Cherokee, Cherokee Six), the flaps had step areas on the inboard ends. When retracted, they were essentially locked in position. When extended to the first notch, they were spring loaded in such a way that they could be pushed down to expose the hinges and linkages for inspection from above. The pre-flight routine would include setting the flaps to 1 notch to facilitate inspection during walk-around, but otherwise keep them retracted until pre-take-off. So a reason for keeping retracted would be to help with boarding.

"On Pipers (Cherokee, Cherokee Six), the flaps had step areas on the inboard ends. When retracted, they were essentially locked in position." -- bizarre.  So there's no problem stepping on the retracted flaps, in the inboard step location?

@quietflyer See this picture. Although the flaps are extended in this image and are not a safe step, you can see the black traction surface on the upper wing around the cabin door, and the same traction surface continues on the flap. It is a safe step when fully retracted.

authors: سجودي البرازيلي


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Is it possible to takeoff without flaps extended?
related questions:

landing wihtout flaps
takeoff with full flaps

My question focus on passengers jets (goes from Embraer E-jet family to  Airbus A380). I don't know why pilots would attempt to takeoff without flaps but lets say they will.
Considering all the other parameters are good enough (no crosswind, long runway, aircraft light enough,...) and the pilots are aware of this unusual takeoff configuration and adapt their action accordingly (including increasing $V_r$ and $V_2$), is it possible for any passenger jet to takeoff and stay airborn (not like the flight spanair 5022) without extending flaps?

As for why it could be because the flaps are inoperative and no facilities for repair exist.

An important aspect to this questions is not whether or not it is possible, but whether or not it is legal, with manufacturer approved procedures and performance data.

@Lnafziger Indeed, but my question do not adress the reasons of such takeoff, only its possibility (I think the reasons require another question)

Also, one other aspect you may wish to consider in this question is whether or not it is possible without slats as well, since they are generally used together and for the same reasons.

This is an opinion-based question, except for a test pilot who might in a position to make no-flap departure in transport class aircraft

@rbp Not if there is a procedure for it.  :)

@rbp as highlighted by other comments, this question is not about the reason of such non standard take off.

@Lnafziger but its not a question about a procedure for a specific airplane, but about "passenger jets" in general, and any answer is speculative

@ManuH i never mentioned anything about reason. this is an opinion-based question and any answer would have to be an opinion.

@rbp I disagree. I asked if it is technically possible, i.e. if it has been done at least one. A possible answer could be "yes, a test pilot has already made it". The answer by hokkuk highlight a possible extension to the question about condition of such takeoff.

I've edited the question to match the comment

The answer could be its yes its been tried on a model aircraft or no so this is a good question

@securitydude5 -- model airplanes take off without flaps all the time

Yes take-off without flaps is possible. The Airbus A300 and Boeing 767 are approved for such take-offs and it is being done regularly. It results in a better climb gradient, especially with one engine out. The one engine out climb gradient is an important and sometimes limiting factor in take-off calculations.
One important constraint is the length of the runway.
Another limitation, perhaps less obvious, is the maximum rolling speed of the wheels. The tires are rated to a limited speed which may be exceeded by a flap-less take-off.

Are you sure the 767 is "no flaps" as opposed to "flaps 1"? Flaps 1 moves the SLATS to the mid position, but no adds no flaps. Flaps 5, 15, 20 add flaps as described. Flaps 25 and 30 move the slats to the second position, and the flaps as described.

@rbp "no flaps" as in "adds no flaps". Slats are required IIRC.

Thanks to DeltaLima for the Answer, I want to add the Fokker 100 to the Planes with no-flaps take-off capabilities. But as said you need a higher take-off speed. also stated in table:

you can make a brick take off without flaps,  if you put enough power behind it, and those jets have enough power,  (as long as the runway is long enough for those particular hunks of metal)
a good question can be added on,  how long of a runway does a 'xxx' need with half load,  with empty load, with full load and fuel,  with broken flaps.

And another follow-on question to that might be "How long will it take the FAA (or other applicable regulatory agency) to revoke the pilot's license after it's discovered that he or she took off with broken flaps?"

Sure.
Whether the aircraft is still on the runway or already airborne when it reaches the airspeed necessary for sufficient lift without flaps is rather irrelevant.

Not entirely irrelevant when you consider the tire speed limitation.

ground effect may help if the aircraft is still on the runway

@DeltaLima That's only relevant to whether it can be done safely and legally, not really to specifically the question of 'can it be done?' :)

The Fokker 70 and Fokker 100 take off with no flaps even on short runways.

Whilst this might be correct, some reference (not WikiPedia) would be good.

Especially since TesterMen Tester's answer from November 2014 already discusses the Fokker 100, with data.

Flaps are a drag device.  Airlines have toyed with the idea of no flap takeoffs for a while.  You simply need higher airspeed for liftoff.  It is completely safe with the correct speed....usually 3-15 knots faster depending on airframe.

As highlighted by a comment in another answer, you should also consider tire limitations when asking for higher speed before liftoff.

Flaps are a "drag device" in the same sense that wings are a "drag device".

You may be able to take off but as soon as you reach a height of 40-50 feet you lose wing in ground effect and will crash unless you have reached actual free air flying speed. Assuming a completely unobstructed route ahead of the runway the plane will eventually get enough airspeed to fly normally without flaps.

Ground effect isn't an off and on thing. You gradually lose the benefits of ground effect with increasing distance to the ground. It doesn't just suddenly disappear.

I think you will find that ground effect drops off exponentially with increasing height and is only 10% by the time a height of 90% of wing span is reached. The other factor is that the plane will stall very easily because of the high angle of attack normally involved in take-off and there is little height to play with to drop the nose back down.


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When should the throttle be closed on a C172 while landing?
during landing training in a cessna 172, would you rather cut the power to idle at beginning of runway threshold, or idle when you're close to ground? I keep having problems where I flare too high or the plane bounce when it touch the ground, flaring is quite difficult.

Related: https://aviation.stackexchange.com/questions/27734/how-can-i-stop-being-too-high-or-low-when-on-base-and-final/27742#27742

What does your instructor say about this? They can see exactly what you are doing and we can't.

Don’t think about flaring, think about holding the airplane off the ground.  Try to keep it from landing for as long as you can.

@Jim has a good point there. One of the tricks to make a good landing ist to let the plane sink close to the ground, but then "tease" it by not letting it land.

When you start to round will depend alot on your angle of descent (obviously you don't want to wait too long) (that's why power on roundouts are a little easier).  Rounding a little higher, but more gently, helps avoid "ballooning" the roundout.  This will come with experience.

@Jpe61 - Yep, I can still remember my instructor (nearly 40 years ago) saying,”Hold it off, hold it off, hold it off…”

Re question posed in title of this question-- for me generally well before turning final.

@Jim Your instructor never stops talking to you, no matter how many years it's been.

Sounds like an improper flaring technique, height perception at the roundout and/or improper final approach speed.
You should always be flying a stabilized approach ie power set to maintain Vref, aircraft configured to land and trimmed correctly to neutralize control pressures on final.  Vref should be at the value recommended by the manufacturer, or not greater than 1.3 times the stalling speed in the landing configuration.  Configured to land, this value is approx. 61 KIAS in a Cessna 172 as indicated in the POH.  You should fly this stabilized condition, on glide path and on centerline all the way down final to the start of the roundout.
You will start the roundout at approx 10 ft above the runway, smoothly pulling power to idle while simultaneously applying back pressure to both arrest your descent and compensate for the loss of lift and elevator authority due to decreased propwash over the wings and tailplane.  Judgment of how high off the ground you are is based solely on perspective.  You need to use you perspective from your entire field of view, not just how the runway appears, to judge height above the surface.  We can practice this in a exercise I will describe below.
The key to our control inputs here is that we want to command the airplane in such a way that it 1) arrests your rate of descent so you arrive at 1-2 feet above the runway with 2) the airplane in the correct attitude to gently make contact with the runway surface on its landing gear.  This takes a bit of practice to develop the hand-eye coordination based on the kinesthetic feel of the airplane and control feedback to get this right.
The best exercise to practice this with is the low approach followed by a go-around.  NOTE WELL: this is a dual instruction only exercise and should never be performed by a student pilot on solo flights.  In this exercise, you will fly a final approach to roundout as you normally would for a landing but not pull the power during the roundout.  This will cause you to enter a low approach, flying in ground effect, down the runway centerline at approx 1-2 feet above the surface.  With power still set for that used on final, the airplane should ‘float’ in ground effect down the runway.  You will continue to fly in this low approach for about 70% the total length of the runway before executing a go-around.   Be sure to note the total length of the practice runway during your preflight and establish with your instructor where you will being the go-around to ensure safe obstacle clearance during the departure back into the traffic pattern.
It’s a good idea to select an untowered airport with low traffic and a long runway available to give you plenty of uninterrupted practice and time in the low approach to get practice in this environment.  Your instructor can recommend some airports nearby that can offer a good training environment to do this at.
This exercise gives a student pilot a good opportunity to safely practice, when conducted on a dual flight with a CFI on board, the following skills

Judging height above the ground by perspective alone.  During this exercise you can call out the with you are at both the proper height for the start of the roundout as well as established 1-2 feet above the runway, and the instructor can offer feedback, whether he/she concurs with your judgment and make corrections, providing you with the correct sight picture in during roundout.(NOTE: prior to beginning a training session using these techniques, a student should carefully study the perspective from the cockpit while lined up on the runway prior to takeoff and mentally photograph this perspective as his/her reference of the correct roundout height.  This is also an excellent technique for tailwheel airplane pilots practicing three point landings because it not only gives the correct height perspective but what the correct 3-point pitch attitude should be at touchdown).
Maintaining height above runway during slow flight and judging whether the aircraft is maintains altitude or whether you are descending towards the runway or ‘ballooning’ back into the air due to incorrect, insufficient or excessive elevator input.
Controlling the aircraft in ground effect during slow flight.  While students are taught slow flight techniques at altitude, the standard exercise does not provide the feel of the airplane in ground effect.
Maintaining directional control, runway centerline alignment and proper attitude in a crosswind during the roundout.
Executing a proper go-around during the roundout and how to mitigate slow speed and/or high AoA hazards in this regime.

Once a student has mastered these techniques, they can attempt to do another low approach to go-around, but once established in the roundout, correctly controlling the airplane, the instructor can smoothly pull the throttle to idle while the student maintains the correct roundout attitude.  The student should be warned prior to this to be ready to add additional back pressure once he/she senses the throttle being reduced to idle power to compensate for a loss of lift from reduction of propwash and prevent the airplane from quickly sinking to a firm landing or wheelbarrowing.  If this is done correctly, and the student maintains the correct roundout attitude and altitude, the airplane should quickly be exhausted of airspeed and gently settle onto the runway on its main landing gear.  We then transition into a high speed ground roll.  With addition practice, the student can learn to coordinate throttle reduction and elevator inputs so they can reduce power to idle at the same time they start the roundout.
One final note, unless you have an emergency, never attempt to force an airplane to land if it does not want to do so.  If, for any reason the airplane won’t settle down onto the runway, the flare balloons excessively or other excessive floating happens just execute an immediate go-around.  If you have fuel on board and all systems are functioning correctly, you have time for another attempt.  Sometimes landings, for whatever reason or other just don’t work out and it’s just best to go around and try again.
Hope that helps as it come from someone who struggled with landings and realized no instructors were teaching these skills worth a damn.

The amount of power during landing a 172 only has to do with your angle of approach to the runway.

on final approach/flare/landing airspeed control is critical

Pitch (trim) controls airspeed, power controls your aiming point
So, if you are doing a 3 degree power on approach at 65 knots try cutting power smoothly as soon as you round to level with the runway at around 10-15 feet.  Then allow speed to decay as you pull the nose up.  After that the plane should almost land itself.

When learning to land "walk before you run"

If you get a good roundout, flaring should be not problem.
When coming in to land, arresting your descent at the proper height over the runway is the number one priority$^1$.  When learning, it makes no sense to overcomplicate this crucial step.  Only chop throttle when you reach 10-15 feet off the runway.  Then flare and land.
Be patient and keep flying it until the nose comes up.  Practicing on a longer runway is very helpful.  You have to know when your plane wants to land.  Forcing it at too high an airspeed can cause bouncing or skipping.
Be ready to go around.  Practicing rounding out and flaring at the proper height will make perfect.  This is why carrying 65 knots gives one a few more precious seconds to get "in the landing groove" before flaring and touchdown.  For now, leave that throttle alone until you are properly rounded and on the centerline.  Land (safely) a few hundred feet further down the runway.  When you get to short field landings, proper rounding becomes more critical.  Work on it with your instructor.
$^1$ at this point most of the altitude energy is gone and airspeed is decreasing.  Ground effect helps some here, but (if you don't go around) one way or the other the plane is going to land.

Power off landings are the same airspeed, the approach angle will be more like 6 degrees to get more "push" from gravity.

My instructor sent me a Rod Machado video on when to flare using visual cues from runway (The Runway Expansion Effect on YouTube). Rod also explains when to reduce throttle.

It first helps to understand perspective which, according to Oxford, is defined as “The art of drawing solid objects on a two dimensional surface so as to give the right impression of their height, width, depth, and position in relation to each other when viewed from a particular point.“ In our case, it is a matter of paying close attention to the relative size of the closest portion of the runway to the furthest point. Thinking of your windscreen as a canvas.
Here are the main visual cues:

Maintain the landing point in a fixed position in the windscreen.

Notice how the beginning of the runway flattens out and widens faster as you approach.

The expansion grows geometrically (increases at an increasing rate) even though your rate of descent is constant.

Once runway is assured, reduce throttle smoothly to idle.

Round out should occur followed by flare when runway perspective rate increase is 10x/sec.

Things to note:

Must have a stabilized approach no faster than 1.3x Vs (velocity of stall)

Review above mentioned video

Watch other landings to understand perspective rate of change

Use a simulator to experiment safely if available

Discuss with instructor and/or other pilots

Hope this helps

Thank you for summarizing the video! I put the link back in (it's not forbidden to link to external sites, we just don't like it if an answer becomes useless without it).

This has become a great answer now, very nicely done!

When I teach in the old taildraggers,(65,75,85 hp) I suggest cutting power on down wind just before turning base.  About every 30 seconds, I move the throttle forward to keep the engine clear and to ensure that I still have power if a go-round is necessary.  It is good practice to do this from time to time in newer aircraft.  If you are flying a Piper single and not using carb heat in the pattern, be prepared for carb ice.
With steady winds, chop the power when you know the runway is ‘made’.  Then just “hold it off—hold it off” till the mains touch down.  The nose should be somewhat high.  Ease the yoke back so that it is full back before the nose touches down.(adjust for gusty winds)

I’ve been planning to do this on a Cessna for a while now (just haven’t gotten the chance because the weather’s been bad for weeks now). Will definitely keep in mind to “warm up” the engine every 30 secs…as taught in forced approach practice.

"Cutting power" sounds harsh, but then again you definitely should be at idle way before rounding out is done.
What I did, Step by step

Establish stable approach
At threshold & about 50ft, start retarding the throttle
Start gently pulling the nose up, keep eyes at the far end of the runway
Throttle closed, keep attitude such that the plane is sinking towards the rwy. Only smooth steering now
As you are a couple of feet from the ground gently add pull to let the plane ease into the ground.

So no sudden cut of engine power at any stage, but a steady pull on the throttle like 1...2...3
I mentioned stable approach, because you are setting yourself up for a mess if you need to constantly adjust power during the final stages of approach. In that case your speed (energy) and approach angle are in a constant state of change, and that state will continue all the way to the touchdown. Or bounce as it often turns out.

• Have a stabilized approach and keep the approach picture throughout a large portion of your final approach. This is where a good landing begins. If your plane’s attitude is too different from what it should be over the aiming point, you’re going to have more troubles managing all the energy and settling the plane down gently.
• Maintain the right airspeed. The POH recommends 55-65 kts. A lot of people aim to be at 60 KTs because it’s halfway and seems ideal: I personally prefer to aim for 62.5 kts. Above 65 is too fast. But over 60, even being at 65 is just fine. The slower you are, the sooner the airspeed is gonna fall when you cut the power but you have to still go through the round out and flare stages- and that’s when the plane hits the ground harder.
• Know your aiming point. I was taught to “cut” the power as I’m “crushing” the aiming point. Please be gentle while closing the throttle: the abruptness of pulling the throttle back is directly proportional to the abruptness of the right yaw you’ll get after that.
• When you flare, be gentle and look at the end of the runway: you should be able to see the runway going down as you pull back. Spend time on dual flights to better your judgement of the right amount of asphalt you should see in your viewpoint as you flare. Or find any other reference that works for you. You can even talk about this on the ground with your instructor.
• It’s paramount that you round out in to CRUISE attitude. Any lower and closer to the ground and you risk porpoising. Any higher of a pitch and you risk a tail strike as you keep pulling afterwards. I’ve had both of these. So speaking from experience.
Personal note: For a large portion of my PPL training I was too scared to wait before reaching a good height above the ground to flare at. So naturally my instructor was a bit frustrated with me not being able to let go of the habit of flaring too high (he hid it well though). And the onset occurred after I soloed and started using full flaps instead of 20 degrees. Try your best to get out of fears like this: that helps. And confront these fears. Acknowledge these and actively work on getting rid of these.
Good luck!


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How to ballpark IAS vs TAS?
Is there some reasonably accurate ballparking formula to allow for quick conversions between IAS and TAS as a function of pressure altitude and difference of ground temperature relative to standard atmosphere? That is,
E.g. "for each 1,000ft subtract X%, for each degree F/C subtract Y%" so that if I'm at 8,000ft and ground temperature is 10 C, I simply subtract 8X% and add 5Y% to get a good approximation.
My idea is to have a quick ballpark to start working with, in case there is no time to start fiddling with the flight computer, or in cases where the error would be insignificant for practical purposes.

Is there a certain range of airspeeds/altitudes you are interested in? The relationships are not linear, so higher altitudes and and supersonic speeds will change the calculation.

I came up with a rough figure of +1.7% of CAS for each 1000ft, and +/-1% for each +/-5 degrees of ISA delta from playing with this calculator.

@SteveKuo For all GA airplanes that I am familiar with, Calibrated and Indicated Airspeed are the same at cruise. It is usually only with flaps and low speed that they differ.

@JScarry Perhaps then question should have been phrased "CAS vs TAS"

Indicated vs. True is a hard one, as every aircraft has an instrument error and a position error. Once calibrated into Calibrated Air Speed however, this document gives CAS as a function of TAS:

All of them more or less straight lines, gradients a function of the altitude.
For temperature: the rule of thumb of density altitude can be used. From this website:

Keep in mind the standard temperature is 15 degrees C but only at sea level. It decreases about 2 degrees C (or 3.5 degrees F) per 1,000 feet of altitude above sea level. The standard temperature at 7,000 feet msl, therefore, is only 1 degree C (or 34 degrees F).

So the procedure is:

Convert pressure altitude with temperature deviation. Ground temperature = 10 °C so density altitude = (10 - 15) * (1000/2) = -2,500 ft
Find TAS at 30,000 ft by subtracting 2,500 ft, then applying the TAS thumb rule of 2% / 1,000 ft: TAS @ 27,500 ft = 27,500 * (1.02/1,000) = 1.55 CAS.

At 30,000 ft and ground temperature of 10°C , TAS = 1.55 * CAS

Good equations for that kind of thing here:  http://www.edwilliams.org/avform.htm#Mach

Ed was working on that formulary in 1980, when I was working with him. He has maintained, and refined, that reference for many years.

The correct notion of "density altitude" is pressure altitude plus (temperature difference * lapse rate stuff), so in this case the density altitude is actually 30000 + (10 - 15) * (10002/) = 27500 ft.

@SOFe Indeed, as calculated in the first bullet point.

@Koyovis the first bullet point claims that density altitude = -2500ft instead of 27500ft.

It computes what to subtract from the density altitude due to temp. 30,000 - 2,500 =27,500.

As @Adam cites, Ed Williams has put together a nice aviation formulary.  So I won't consult it in this quick answer, which I use as a rule of thumb only.
To ballpark TAS, for every 1000ft increase CAS (or IAS) by 2%.
That is a ballpark, and it, for example, will not work when you are flying a U-2 at 70,000 feet.
Addendum: Many aircraft have a sliding wheel on the ASI (Airspeed Indicator) which will allow you to move a TAS scale around the bezel of the instrument. For anyone wanting more precision than a ballpark rule, you might consider: http://www.dtic.mil/dtic/tr/fulltext/u2/a280006.pdf

Although it is always quoted as TAS = CAS + 2% for every 1000FT, wouldn't it be more accurate to say TAS = CAS + 2% for every 1000FT of pressure altitude? After all, we are correcting for variations from the ISA.

@jumblie, in what circumstances would you expect a significantly non-linear pressure lapse rate?  In general above some altitude, say 10,000 feet the 1000 foot guidance gets substantially off, because of the atmospheric density distribution with altitude.  So the refined use of pressure altitude is unnecessary and implies more accuracy in the rule of thumb than really exists.

I wasn't necessarily thinking non-linear (though use of pressure altitude would mitigate that too), but anything where the pressure lapse rate deviates from the norm such as extreme temperatures.

Well the question was for a ballpark, and I figure that a ballpark figure is something one can easily do in their head. A student can do the calculation in their head, and it is reasonably accurate.  But adding the refinement of pressure altitude increases the complexity. If one was creating a calculator for TAS and CAS, then there are several refinements that can be made, and I agree with you on pressure altitude for that, and the NASA paper cited develops a refined model.  We just diverge on the reliance of pressure altitude for a ballpark rule.

Thanks @Mongo for clarifying that pressure altitude would indeed be more accurate albeit less practical. I agree with your thoughts. The reason I brought it up in the first instance was to check that my understanding of what was going on was correct.